Here is a an old post that might clear up some misconceptions.
First let’s clear up the "Big Boy" question. It is like calling all tissues Kleenex.
To be specific there are several variations of the "Big Boy" solenoid. The solenoid is a very simple electric switch controlled by a 12-volt source. See, even the Big Boy is not always called that. As an electronic tech I will refer to it as the relay. How's that for confusion.
So, Big Boy, Solenoid, Relay, and often because of the mfgr of these they are called Trombetta relays.
How these relays are activated can be complex or as simple as sending 12 volts and ground to them.
You will notice on your relay in the battery compartment that there are two wires. One is likely purple. In any case one is ground and the other is switched 12 volts. The purple wire is the switched voltage in most cases.
Again, 12 volts is sent to the purple wire and the big relay goes clunk and makes contact between the large wires thus combining the batteries.
That relay under normal working conditions will get very warm or even hot to the touch if it runs for several hours. For example, if things are working properly and you are using shoreline it will be working 24/7 and get hot to the touch. This is normal.
So, the heart of the system is that relay on the back being controlled by 12 volts. How and where that 12-volt control comes from can be the tricky part to understand...
We will get to the BIRD part shortly.
First, if the engine is running and after a few minutes of warming up and the grid heater stops loading down the battery voltage on the chassis batteries should rise enough to engage the BIRD device and send the 12 volts to the relay. This is the “Delay” part of the bird. It would not make sense to combine the batteries without the alternator having brought up chassis batteries a bit.
So, let’s take the situation where your chassis batteries have been run down for some reason. You left the radio on for two days etc.
Pressing and holding the Aux start button bypasses the fancy “BIRD” circuitry and sends 12 volts directly to the relay in the battery compartment. This combines the battery banks just as a set of jumper cables would.
Part 1:
Part 2.
Now that we understand how the relay functions let’s begin managing when it is turned on. We already discussed how the “Aux” start button controls it. That is a manual function and can easily be tested by listening to the relay as someone presses the button. If no clunk is heard, then the relay is most likely already activated. To test this without a meter, simply remove the purple wire. Wear some gloves because there will be a tiny static like discharge that can surprise you. If you have a meter you can measure the voltage on the little terminals. You can also carefully feel the relay to see if it is warm or hot IF it has been engaged for some time.
Here is how the Bi-Directional Relay Delay functions to control that purple wire.
Keep in mind the “DELAY” part as you are troubleshooting. It can take a couple of minutes for it to activate. No sense in combining batteries if the first bank being charged is not up enough.
Parked with nothing running and no shoreline the bird is asleep. No combining. However, if you have solar that is indeed a charging source and may be enough to activate the bird.
Now you plug into shoreline. The little circuit board in the front run panel wakes up and sees that your converter is doing a fine job of charging the coach batteries. The converter is connected directly to them via cutoff switches and fuses or circuit breakers. The BIRD now sends voltage to the relay and engages it, thus combining the batteries.
Time to leave, shoreline is disconnected so the BIRD drops out the control voltage. You fire up the big engine and in a couple of minutes the BIRD sees a nice alternator doing its thing. Time to send voltage to the relay again.
We have just discussed the BI-directional part of the BIRD. This demonstrates how it can work from one set of battery banks to the other depending on which one is getting the charge.
Arriving at your favorite boon docking place you decide to fire up the generator to run some heavy loads. This generator is the same thing as having shoreline power, so it works as described before.
So, you spend the weekend without the generator running anymore and enjoy the quiet along with
some tv and perhaps running the microwave on the inverter. Inverter of course not the converter. They can be combined in one box and I can address that more if needed. Inverters are real battery hogs so now you have a very low set of batteries.
You are packed, and the big engine is started, thus providing a big enough voltage to activate the BIRD. You are now charging 6 batteries or more and the alternator is straining. This is rough on the alternator, so you follow the manufacture and my advice and fire up the generator so the “converter” comes alive and supplies voltage to the dead batteries.
At this stage you have two sources of charging voltage. The converter via the generator, and the alternator. This poses a bit of an issue with the alternator and can cause it to show a fault. However, the BIRD is rather smart and knows the engine is running along with the generator. With these two competing charging sources it simply turns the Relay off and allows each battery bank to be charged separately.
Now that you understand how it works, here is a scenario that could get you home if your alternator dies. Just start the generator and prop the AUX button up so it forces the banks to combine. Since your alternator is offline there will not be any conflicts. This will easily get you any distance you wish to travel.
Of course, you could put a small jumper from 12 volts to the purple wire and do the same without pushing the button.
Now you understand how, when, and why the relay is activated. What you don’t know is if it is actually working. The relay is a very simple device inside. It is a magnet that pulls a contact across the two large wires connected to it. Very often these contacts burn and fail to make contact. This can be intermittent and drive a technician crazy.
If the relay is activated there is either a charging source or the AUX button is being held down. When it is activated it is like placing a nail across the two large terminals. So you should see the exact same voltage on each of the large terminals and at the batteries. If you do not see the exact same voltage across the large terminals, then the relay is bad.
This can be a bit tricky if both banks of batteries have been fully charged and the relay is making contact but a poor one. You will see the same voltage on each large terminal and think the relay is ok. You need to run one bank down a bit so there will be lots of current trying to charge for an accurate assessment.
If you are having one bank of batteries low after having a charging source for several hours and you find voltage on the purple wire then the relay is bad.,
These relays are a known source of problem and I had a heck of a time understanding how the system works. I am a senior certified electronic tech and extremely qualified to work on such a simple system as long as I know how it is supposed to function. I did not have that information in 2008.
Replacing the relay is easy and cheap as things go on these things. Be sure to turn both battery banks off and I would highly recommend removing the negative leads of each bank for additional safety. You can weld with these battery cables so do be careful.
Once you replace the relay do yourself and us a favor by taking it apart and posting pictures of the insides.
The actual BIRD control board is a common failure item too but has become difficult to source. I found one on e-bay.
Don’t despair if that is the problem because there are plenty of other solutions available.
There is another relay that has nothing to do with the charging system but gives plenty of problems. It is the “Salesman” switch/relay. It is the relay controlled by the switch by the door or nearby. The relay is in the box up front near the BIRD board. The BIRD board is a small board. The relay is on the bottom left.
This thing controls lots of functions and can go bad anytime. Simply bypassing it with a jumper is easy and permanent fix if it fails.
H/R Endeavor 2008
Ford F150 toad >Full Timers
Certified Senior Electronic Technician, Telecommunications Engineer, Telecommunications repair Service Center Owner, Original owner HR 2008
